Bleaching or whitening of pulp is typically accomplished by chemically altering and/or removing colored matter in the pulp and imparting a higher brightness thereto. Chlorine-based chemicals such as chlorine, chlorine dioxide and hypochlorite have been used in pulp bleaching for many years and continue to be used for removing lignin and bleaching the pulp to high brightness. Growing environmental concerns have led to an increase in the use of chlorine dioxide and nonelemental chlorine agents such as oxygen, peroxide and/or ozone in bleaching processes.
Waste streams containing sodium sesquisulfate result from the production of chlorine dioxide for use in bleaching pulp. The increasing use of chlorine dioxide in pulp mills thus makes recovery of the acid and sulfate values of such waste streams of considerable economic importance since the components recoverable from the waste streams may be recycled for a variety of uses.
One process for recovering sodium sulfate and sulfuric acid from a stream containing sodium sesquisulfate is described in U.S. Pat. No. 5,116,595 and is commonly referred to as the "R10" process. The '595 patent is specifically directed to treatment of a waste stream obtained from a process for generating chlorine dioxide for use in the pulp mill commonly known in the art as the "R8" process, and described in U.S. Pat. No. 4,081,520.
In accordance with the method described in the '595 patent, a slurry containing sulfuric acid and sodium sesquisulfate is filtered, contacted with water, and thereafter filtered again to separate the resulting sodium sulfate precipitate from the sulfuric acid solution. One significant disadvantage of recovery processes of this type is that they require filtration techniques, typically vacuum filtration techniques, and the concomitant equipment and operational costs associated with these filtration steps. However, such filtration techniques have been considered vital to achieving sulfate solids of satisfactory yield.
Accordingly, it is an object of the present invention to provide a method and apparatus for treating a sesquisulfate-containing stream to recover valuable constituents thereof.
It is an additional object of the invention to provide an apparatus and method for treating a sesquisulfate stream to recover acid and sulfate values of the stream.
Another object of the invention is to provide an apparatus and method for treating a sodium sesquisulfate-containing waste stream to recover a sulfuric acid solution and a sodium sulfate solids.
A further object of the invention is to provide an apparatus and method of the character described which avoids or limits the need for mechanical filtration such as vacuum filtration.
Still another object of the invention is to provide an apparatus and method of the character described which is uncomplicated and economical as compared to conventional methods and which does not compromise the quality or quantity of the yield.